Not Applicable
Not applicable
The present invention pertains to treatment of drinking water and, in particular, to the use of chlorine dioxide, chlorine, ammonia and mixtures thereof in various stages of currently used drinking water treatment processes.
Chlorine (Cl2), chlorine dioxide (ClO2), ozone (O3) and monochloramine (NH2Cl) are the chemical disinfectants most commonly used in treating drinking water. Sodium hypochlorite (NaOCl) solution is sometimes used in place of chlorine gas to produce essentially the same chemical species in the drinking water. These species (Cl2, HOCl, and OClxe2x88x92) are individually and collectively referred to as xe2x80x9cfree available chlorinexe2x80x9d, or xe2x80x9cchlorinexe2x80x9d. The term xe2x80x9cChlorinexe2x80x9d, as used in this document includes chlorine gas and/or any of the various species formed by dissolving chlorine in water. Monochloramine is created by mixing chlorine and ammonia, typically by injection of chlorine into water containing an excess amount of ammonia, i.e., more than two moles of ammonia per mole of chlorine injected.
In addition to the foregoing, other chemicals, such as potassium permanganate (KMnO4), also serve as oxidants in drinking water. Oxidants can aid in removal of dissolved metals and destruction of some problem organic compounds.
In a typical drinking water-treatment plant, raw water is drawn from a lake, reservoir, river, stream, underground aquifer, or other body of water. Various chemicals are added to the raw water to oxidize contaminants, achieve disinfection and/or enhance removal of solids during subsequent process steps. The water is then subjected to various solids-removal steps that typically include coagulation, sedimentation and filtration. Alternatively, solids removal may be achieved through other processes, such as dissolved air flotation and membrane separation. Following solids removal, the water typically flows to a finished water storage facility and then to a distribution system.
The extent of disinfection in drinking water is a function of the concentration of the disinfectant and the time the disinfectant is in contact with pathogens. A common expression of the level of disinfection achieved is the term xe2x80x9cCxTxe2x80x9d (concentration C, multiplied by contact time T). The concentration of a disinfectant declines as it reacts with contaminants in the water. The degree of disinfection at a given CxT level is a function of many variables, including temperature and pH. There are complex rules for calculating CxT, and there are governmental guidelines for, and regulations mandating, the levels of CxT required for safe drinking water. Each disinfectant has a different set of CxT values necessary to achieve the levels of disinfection required for each type of pathogen, (e.g. virus, bacteria, encysted parasites) under various treatment conditions.
Historically, drinking water treatment plants have used chlorine as an oxidant and disinfectant. Chlorine, however, reacts with organic compounds in the water to produce halogenated by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs). There is increasing evidence showing that these compounds are probably carcinogenic. There is also increasing evidence that these compounds may cause other adverse health effects, such as an increase in the incidence of miscarriage among pregnant women. Government regulations have continued to lower the maximum allowable levels of these compounds in drinking water.
In contrast to chlorine, chlorine dioxide does not chlorinate organic compounds to a significant degree; rather, it oxidizes material through a variety of largely, one-electron transfer mechanisms. When added to natural water for the purposes of oxidation and/or disinfection, it does not produce halogenated byproducts (e.g., THMs) to any appreciable extent. Compared to chlorine, chloramines have less oxidation potential. Because chloramines do not react as quickly as chlorine with organic material, the amount of halogenated by-products produced is significantly lower than with chlorine. Ozone does not produce THMs or HAAs, but it can produce other problematic by-products, such as bromate ion, depending upon the composition of the water.
Chlorine, chlorine dioxide and ammonia, as gases or aqueous solutions, can be used in various combinations in the process of treating drinking water. Chlorine dioxide can be used as an oxidant and as a disinfectant. For either purpose it can be introduced into early or later stages of the water treatment process.
Chlorine dioxide for treatment of drinking water is typically produced by reaction of chlorine and sodium chlorite, as taught in U.S. Pat. No. 5,110,580. Chlorine dioxide can be produced at lower cost through other processes such as acidification of sodium chlorate (NaClO3). However, this lower-cost chlorine dioxide is produced as a mixture with chlorine. In the prior art, it was necessary to separate the chlorine from the chlorine dioxide and recycle the chlorine using complex and expensive process steps, e.g., by the Day-Kesting Process described in Ullman""s Encyclopedia of Industrial Chemistry 5th ed. 1986, Wersheim, New York, N.Y.
Chlorine, chlorine dioxide, and monochloramine are used in various ways to oxidize contaminants and to disinfect drinking water. Chlorine dioxide and chloramine are used individually in drinking water, but not injected together. Typically, chlorine dioxide is injected early in the treatment process, while chloramine is used near the end of the treatment process.
The present invention has, at its core, the use of a mixed stream of chlorine and chlorine dioxide, sometimes with ammonia, to disinfect and preoxidize drinking water while minimizing production of THMs and HAAs.
All three components (chlorine, chlorine dioxide and ammonia) can be introduced together into the raw water. The chlorine dioxide provides rapid disinfection and, in the typical application, is consumed almost as rapidly. The chlorine and ammonia combine to form monochloramine, which provides slower but long-lasting disinfection. Monochloramine typically will persist in the water throughout the process and into the storage and distribution of the clean, potable water.
Thus, in its broadest aspect, the present invention is a method of treating water to produce residual monochloramine and chlorine dioxide in the water, comprising the steps of: injecting a mixture of chlorine and chlorine dioxide into the water together with ammonia, the ammonia being present in an amount sufficient to produce residual monochloramine, with substantially no chlorine in the water.
In another aspect, the present invention is a method for treating water as it proceeds from a source to a storage or distribution facility, comprising the steps of: injecting a mixture of chlorine and chlorine dioxide into the water at a location between the source and the storage or distribution facility; and injecting ammonia into the water either upstream or downstream of the location where the chlorine and chlorine dioxide are injected into the water, the ammonia being injected in an amount to substantially react with the chlorine, whereby the water in the storage or distribution facility contains chlorine dioxide, monochloramine and a negligible amount of chlorine.
Therefore, yet another aspect the present invention is a method for treating water using a stream containing chlorine and chlorine dioxide, comprising the steps of: separating the chlorine from the chlorine dioxide to yield a stream of chlorine and a stream of chlorine dioxide; using the stream of chlorine dioxide to pre-oxidize a stream of raw water prior to subsequent steps for removal of solids; and using the stream of chlorine to disinfect the water after some solids have been removed from the water, but prior to storage for distribution.
Since chlorine dioxide decomposes in sunlight, application of the chlorine dioxide at the entrance to the raw water main can sometimes provide a long, dark, well-mixed vessel (i.e., the raw water main) where the chlorine dioxide can effectively react with contaminants.
A further aspect the present invention is a method for treating and disinfecting raw water comprising the steps of: introducing a mixture of chlorine, chlorine dioxide and excess ammonia into the raw water to provide disinfection of the water by chlorine dioxide, and to create monochloramine by reaction of chlorine and ammonia; passing the raw water through further treatment steps wherein solids are removed and whereby residual chlorine dioxide is consumed prior to filtration; and collecting a finished potable water containing residual monochloramine to provide residual disinfection of the finished water.
In some situations, it is not economical to disinfect raw water using chlorine dioxide because high levels of contaminants in the raw water consume too much chlorine dioxide before adequate CxT credit is achieved. Some disinfection is needed throughout the plant to suppress biological growth in basins and filters. In these cases, chlorine and ammonia may be fed into the raw water. The resulting chloramine persists through the entire plant process and serves to suppress biological growth, without producing any appreciable amount of THMs and HAAs. Substantial amounts of the contaminants in the raw water are removed during the solid removal steps, after which a relatively smaller dose of chlorine dioxide may be applied to the filtered water to achieve disinfection. This may or may not be done in conjunction with feeding a relatively small dose of chlorine dioxide into the raw water to help to remove metals and, to aid in coagulation/sedimentation.
A further aspect of the present invention is a method of treating contaminated raw water comprising the steps of: a) introducing chlorine and ammonia into the raw water to produce residual monochloramine, which suppresses biological growth in the water as it proceeds through subsequent processing steps; b) passing the water from step xe2x80x9caxe2x80x9d through solids removal processes; and c) disinfecting the water after solids removal and prior to storage with one of: 1) chlorine dioxide; 2) a mixture of chlorine dioxide and chlorine; or 3) a mixture of chlorine dioxide, chlorine and ammonia.
Still another aspect of the present invention is a method of treating raw water using streams of chlorine dioxide, chlorine and ammonia comprising the steps of: a) introducing chlorine dioxide into raw water by injection into a raw water main; b) subjecting the raw water containing chlorine dioxide to solids removal processes; c) introducing additional chlorine dioxide into the water as it is withdrawn from the solids removal process and conducted to finished water storage; and d) introducing chlorine and ammonia into water withdrawn from storage for distribution to users to provide monochloramine in the water by reaction of chlorine and ammonia.